Automatic repeat request (ARQ) is an error control mechanism utilized in many communication systems. When a packet error is detected at the receiver, which is usually done by cyclic redundancy check (CRC) codes, an automatic repeat request acknowledgement is sent by the receiver back to the transmitter. The transmitter then retransmits the packet. Hybrid ARQ (HARQ) is a variation of a conventional ARQ system that utilizes forward error correction codes (FEC) in the system to further improve the system performance. HARQ is widely used in real communication systems, such as High Speed Packet Downlink Access (HSPDA) or High Speed Packet Uplink Access (HSPUA), IEEE 802.16 (Worldwide Interoperability for Microwave Access (WiMAX), and 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE).
The various embodiments disclose herein relate to wireless communications networks, and more specifically, relate to Multiple-Input-Multiple-Output (MIMO) communications networks. It is general understanding that a wireless station or device may be either a mobile terminal or a fixed terminal such as a base station (BS). A typical MIMO network is comprised of a BS—with multiple antennas and multiple mobile stations (MSs), at least one of which has multiple antennas. Assuming that there are M antennas at the BS and N antennas at one of the MSs, there is an MxN MIMO channel between the BS and the MS. Conventional communications systems have recently introduced MIMO systems with multiple transmit/receive antennas to further improve the capacity of the communications system. MIMO systems that employ Orthogonal Frequency Division Multiplexing (OFDM) are utilized in 3G systems (i.e., third generation of mobile phone standards and technology) and beyond 3G systems, such as IEEE 802.16 d/e (WiMAX), 3GPP LTE, and 3GPP2.
Spatial, temporal, and frequency diversity techniques can be considered and exploited in general MIMO-OFDM systems. Such diversity techniques improve the reliability and/or throughput of a wireless communications system by utilizing two or more communication channels with different fading or changing characteristics to transmit messages. Diversity techniques play an important role in combating fading, co-channel interference, and avoiding error bursts. Diversity techniques allow the receiver to take the average of the signals received from a “bad” channel and a “good” channel to further improve link performance of the communications system.
There is an ever-increasing demand on mobile wireless operators to provide voice and high-speed data services. At the same time, operators want to support more users per BS in order to reduce overall network cost and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities have become very attractive. One way smart antenna technology has addressed this problem is by employing advanced signal processing techniques called beamforming (BF). Beamforming technology has found its way into all the major wireless standards including 3GPP, 3GPP2, IEEE 802.16, and IEEE 802.11 systems.
Beamforming techniques utilize multiple antennas or antenna arrays to control the direction of the transmitted signal. Using beamforming, each user in a wireless communications system is adaptively assigned a particular beam pattern associated with the current channel condition associated with that device. The instantaneous or current channel conditions may be determined by measuring the channel state information (CSI), which is information about a current signal channel in the wireless communications system. CSI is represented by the current channel matrix H, a mathematical construct that represents a MIMO channel in the communications system. The MIMO channel may be modeled according to the following equation r=Hx+n, where r is a vector representing receiving signals; H is a matrix representing channel characteristic; x is a vector representing transmitting signals; and n is a vector representing channel noise. The values for r, x, n, H are usually not constant. The system usually requires some information regarding H to determine what information was sent from the transmitter or to enhance the system performance, including increasing transmission speed. When the CSI is known both to the transmitter and the receiver, the beamforming technique is optimum in terms of capacity of the wireless communications network. When the CSI is not known, beamforming is not effective and diversity techniques (for example, the Alamouti scheme) can be employed to improve the reliability of a message signal by utilizing two or more communication channels with different fading characteristics. When the CSI is partially known, however, it is not clear which techniques are optimum.
The CSI at the transmitting device is usually estimated based on the signals received from itself or another device. In real communications systems, the CSI at the transmitting device may be obtained from a feedback channel in Frequency Division Duplex (FDD) systems (close loop) by utilizing the received signal at the receiver site, or estimated based on receiving signals from a reverse channel and utilizing the reciprocal property of the channel in Time Division Duplex (TDD) systems (open loop). In both cases, there is a delay, such as a feedback delay in FDD, or a delay between current channel and the channel used for CSI estimation in TDD.
In HARQ systems, it is very possible that the CSI at transmitting device is not available for each HARQ retransmission. However, it is assumed that the CSI is always available for the first transmission. In these cases, for the HARQ retransmission, the CSI is partially known at the transmitter when channel conditions change slowly, or may be totally unknown when the channel conditions change quickly. In another words, the HARQ transmitter might have some aged CSI which is used in the first HARQ transmission, or in any previous transmission, and the transmitter needs to determine whether this CSI is suitable for a current HARQ retransmission. Therefore, there is a need for techniques that can employ partial or no CSI knowledge of the channel to enhance the performance of a wireless communications system.